Artificial or prosthetic devices for replacing defective joints in humans, particularly the hip joint, have been the subject of extensive research and development efforts for many years. In total hip arthroplasty, the most common adult reconstructive hip procedure currently performed in the U.S., a metallic femoral component is typically inserted into the natural medullary cavity of the femur. Simultaneously, an acetabular cup, usually of highdensity polyethylene, is inserted into the acetabulum.
A typical prior art femoral component 2 is shown in FIG. 1. The component 2 is an integral metallic component having a head 4, a neck 6 and a stem 10 having a medial side 11 and a lateral side 13. There is usually a collar 7 between the neck 6 and the stem 10, The medial extension of the collar 7 is the platform 8. The stem 10 has a proximal end 19 and a distal end 20 which ends at the tip 22. Various means of measuring such femoral components are used. The neck length 12 is measured from the center 18 of the head 4 to the base of the collar 7. The head-stem offset 14 is measured from the center 18 of the head 4 to the line 22 through the axis of the distal part 20 of the stem 10. The stem length 16 is measured from the medial base of the collar 7 to the tip 22 of the stem 10. The angle .alpha. of the neck 6 is measured by the angle at the intersection of the line 24 through the center 18 of the head 4 and the neck 6 with another line 26 along the lateral border of the distal half 20 of the stem 10.
The femoral component may be made of any strong inert material. Materials which have been used in the past on such components include stainless steel, chromium cobalt molybdenum alloy (Co--Cr--Mo), titanium, or a combination such as Co--Cr--Mo with a ceramic head or titanium with a cobalt-chromium or ceramic head. It may also be made of isoelastic polyacetate.
The head diameter is usually either 22, 26, 28, 32, or 38 mm with a neck length of 30-42 mm. The cross-section of the neck may be round, oval, or trapezoidal. The collar itself may or may not be present. The surface of the stem may be polished, dull, pre-coated with cement, press-fit, or have a porous-metal coating. There may or may not be fenestrations in the stem. The proximal third of the stem may be curved or angulated. The stem may be sabre-shaped, tapered, have a straight lateral edge or an anterior bow or a wide proximal third. The head-stem offset is generally 38-45 mm and the length is generally 12-18 cm or longer. Sometimes the femoral component is made as a modular system with a tapered metal post on the stem to mate with a head component that makes for different neck lengths and diameter of heads made of cobalt-chrome or ceramic. Reference is made to Calandruccio, R. A., "Arthroplasty of Hip" in Campbell's Operative Orthopaedics Vol. 2, St. Louis, C. B. Mosby, 1987, chapter 41, pages 1213-1501.
A major problem from which most prior art femoral components suffer is stability of the component in place. Lack of complete stability can cause pain, failure of the artificial hip, fracture of the femur, or various other problems. Many attempts have been made to avoid such problems and add stability. One such attempt is the use of grouting medium or bone cement to fix the femoral component to the bone. In this case, bone is cleared from the medullary cavity to produce a larger space than required for the stem. Grouting material is inserted to fill the gap between the bone and the stem, as a means for fixing the device and as a means for load transfer between the device and the remaining bone.
While such a method is advantageous in that accurate insertion into the bone is not required and immediate mechanical fixation can be achieved leading to early weight bearing and rehabilitation, many disadvantages result from the inherent weakness of the cement which is exacerbated by poor distribution and/or contamination by blood during surgery.
Efforts have been made to fix implants without the use of a grouting medium, in which case it is important that an accurate bone resection be performed. The femoral component must be selected to give the tightest fit possible to provide a mechanically stable support for physiological loading.
Sometimes the surface of the implant is treated to provide a porous or roughened structure which acts to promote bone tissue growth around the implant, further stabilizing the femoral component with respect to the bone.
A major advantage of the latter system is the absence of cement or grouting medium, thus eliminating the long term inherent weakness and the short term toxic effects of these materials. The disadvantages are numerous. First, these stems have the added requirement of a sufficiently tight fit to prevent motion between metal and bone. Accurate bone resection customized to each type of available implant is difficult to achieve and often results in some initial looseness or lack of support. The implant will subsequently migrate to a more stable position, which may not be the ideal orientation for proper function of the femoral component. The requirement for a tight fit increases the possibility of fracture of the femur during insertion. Additionally, the patient must avoid bearing full weight on the hip for approximately six weeks to allow for bone formation.
Treatment of the implant to form the porous or roughened surfaces may cause local stress sites in the implant which significantly increase the risk of fatigue fractures. Further, a considerable time is required for bone tissue ingrowth and stabilization of the implant to occur. This is a significant detriment to early patient rehabilitation. Additionally, surface treatment exposes a greater surface area of the implant, increasing diffusion of metal ions which are associated with an increased risk of toxic or pathological effects.
The implant's stem may weaken from improper stress loads or decreased fatigue strength due to surface treatment. If this happens, the stem may bend or fracture, requiring its removal, which is particularly difficult if significant bone growth has occurred.
Various efforts have been made to design a femoral component hip-endoprosthesis that can be implanted in the medullary canal in such a way as to provide implant stability without resorting to surface treatment. Some such efforts are directed to creating an isoelastic femoral component which is adaptable to the shape of the cavity created for the prosthesis in the femur and thus transfers the load from the implant outward to the bone surrounding the femoral component in the medullary canal. See, for example, U.S. Pat. No. 4,743,263. Other implants use stepped projections (U.S. Pat. No. 4,031,571) or fixation wires (U.S. Pat. No. 4,530,114) to impose tensile forces on the lateral side of the femoral component in the medullary canal. This is reportedly done to anchor the femoral component while taking advantage of the natural conditions of the bone.
Other efforts to stabilize implants have been directed to adding pins or studs (U.S. Pat. No. 3,896,505), wing-like extensions to prevent rotation of the shank (U.S. Pat. No. 4,664,668), plates to provide anti-rotation stability for the implants (U.S. Pat. No. 4,904,269) and anti-rotation fins (U.S. Pat. No. 4,936,863). All of these efforts are directed to preventing the femoral component from rotating inside the medullary canal after insertion as force is applied to the implant by the patient returning to his or her feet.
Most of these implants suffer the disadvantage of prosthesis dislocation and bone fracturing due to improper force distribution on the femur.
Prior to the present invention, all implants have been designed based on the conventional assumption that the lateral femur is under tensile stress when unilateral loading forces are applied to the femur head. This assumption is based on the standard model for describing the biomechanics of the human hip described in the classic work of Koch, published in 1917 (Am. J. Anat. 21:177, 1917). He determined that the medial aspects of the femur are under compressive load during unilateral load, such as during a stride, and most of the lateral cortex is under tensile loading. In Koch's model, most of the force generated within the hip is attributed to the load of the abductor muscles, anatomically defined as taking origin from the lateral aspect of the iliac crest of the hip bone and inserting on the greater trochanter of the femur. Thus, the superimposed body weight creates a lever across the head of the femur, which serves as a fulcrum, with the body weight force being balanced by the abductor muscle force. This model leads to the conventional wisdom that the lateral aspects of the upper femur are under tensile loading.